Conventionally, spark plugs which ignite air-fuel mixtures by spark discharge have been used for ignition plugs for internal combustion engines of automobiles. In recent years, higher power outputs and lower fuel consumptions have been demanded of such internal combustion engines. Because of this, efforts have been made to develop plasma-jet spark plugs that can ignite leaner air-fuel mixtures which burn out quickly and whose ignitable limit air-fuel ratios are higher.
For example, Japanese unexamined patent application publication No. JP-A-2007-287666 describes a related art plasma-jet spark plug. The related art plasma-jet spark plug has a structure in which a cavity having a small capacity is formed as a discharge space by surrounding the periphery of a spark discharge gap between a center electrode and a ground electrode by an insulator.
The related art plasma-jet spark plug has been manufactured by taking, in general, the following steps (1) to (3). (1) A plate-shaped ground electrode in which a through hole is formed in a center, is press fitted in a ground electrode mounting portion provided at a leading end of a metal shell with a predetermined fitting tolerance. (2) The metal shell and the ground electrode are laser welded together. (3) An insulator in which a center electrode is built in advance is held within the metal shell to which the ground electrode has been welded by the insulator being crimped to a predetermined engagement portion.
In the manufacturing method described above, however, in the step (3), there was a case where when the insulator was made to be held within the metal shell, a shift in position, or “position error,” occurred between the center axis of the through hole in the center of the ground electrode and the center axis of a cavity provided on the insulator. As this occurred, there was concern that spark discharge was performed locally, resulting in a phenomenon in which the ground electrode became worn locally. In addition, when the center axis of the through hole in the center of the ground electrode shifted from the center axis of the cavity provided on the insulator, there was concern that part of the cavity which functioned as a discharge space was closed by the ground electrode, and as this occurred, a quenching action was caused, resulting in a fear that the igniting performance was reduced.